1. Field of the Invention
This invention relates to counteracting the undesired roll that is produced in airborne vehicles equipped with side force generators that utilize sideslip as their primary turning maneuver.
2. Description of the Prior Art
Unlike conventional airplanes that use the roll-to-bank maneuver to achieve directional change, many airborne vehicles--such as missiles or target drones--utilize the sideslip maneuver. Sideslipping or "skidding" is achieved by inducing a yawing moment by deflection of tail fins located near the aft (rear) end of the vehicle. Vertical tail fins act much like a rudder on a conventional aircraft As shown in FIG. 1b, deflecting the vertical tail fins 20 (only the top vertical fin is shown) clockwise (trailing edge to the left) induces a left yawing moment, resulting in a right sideslip with--by definition--a positive sideslip angle .beta.. This sideslip angle allows a net lateral side force, augmented by a side force generator, to be generated, producing a turning maneuver. The vehicle moves in the direction of the velocity vector V while the nose is displaced to the left. If, as shown in FIG. 1c, the vertical tail fins 20 are deflected counterclockwise (trailing edge to the right), a right yawing moment will be produced with a resultant left sideslip and negative sideslip angle .beta..
There are some significant advantages for airborne vehicles to use sideslip for the turning maneuver. Since conventional ailerons on the wings may be eliminated, a simple, one-piece, pivoted, top-mounted wing may be used that conforms to the constraints of the launch device. Further, many of these vehicles carry some type of gyroscopic control device which has to incorporate movement in the X, Y, and Z axes. If the vehicle can be constrained about the X axis; i.e., wings level with no roll, then the computations of the gyroscopic control unit can be reduced, possibly resulting in improved accuracy.
One inherent and long recognized problem with utilizing sideslip is the aerodynamic fact that a nose right yaw--which causes a left sideslip--produces a right roll. A nose left yaw which causes a right sideslip--produces a left roll. The reason for this is the well known dihedral effect, especially pertinent to high wing aircraft. Since the goal is to achieve sideslip without roll, the inherent yet undesired roll must be eliminated to achieve optimum conditions. The only control devices that have been traditionally available to counteract this undesired roll have been--in the absence of ailerons--the horizontal and vertical adjustable tail fins. By asymmetrically deflecting these tail fins in much the same way as wing ailerons, a counteracting rolling moment can be produced. The problem, however, has been that due to their limited size--limited, for example, by launch tube constraints--these horizontal tail fins have not been able to produce a large enough rolling moment to counteract large undesired roll caused by large amounts of sideslip.
Successful attempts have been made to reduce the undesired roll that results from sideslip and can not be corrected through use of the tail fins. Several of these attempts have used side force generators. The patents to Stiklorus and Kavlor teach the basic concept of a side force generator projecting from the bottom of an airborne vehicle. In Stiklorus (U.S. Pat. No. 4,145,017), the side force generator--or keel fin--is fixed in the extended position and ejected at a predetermined time during flight. This keel fin is designed t compensate for the effects of yawing moments and crosswinds during the launch phase of flight. In Kavlor (French patent no. 496,758), the side force generators are spring biased to the extended position and deployed immediately after launch from a launch tube.
Friedal et al. (U.S. Pat. No. 4,601,442) teaches the use of side force generators which are movable from a pre-launch retracted position to a fully extended in-flight position and serve as lateral stabilizers during cruise flight. These stabilizers are rotatably mounted within the fuselage of the airborne vehicle and are deployed to the fully extended position via pre-stressed compression springs. Except for the brief instant that it takes these side force generators or stabilizers to rotate into a fixed position that is essentially perpendicular to the wings, the lateral stabilizers are either recessed within the fuselage or fully extended. No appreciable change in the lateral rolling moment is caused by any transient position of the lateral stabilizers as they are rapidly extended to their spring-loaded, fully deployed position. Once extended, the side force generators or stabilizers taught in Friedal et al. can not be adjusted further.
The consistent problem with all of the prior art side force generators has been their fixed position and resultant fixed aerodynamic characteristics. While effective in certain flight regimes, these fixed structures are often ineffective in others.